Semicontinuous reactors

Manu B, Chauhari S (2003) Decolorization of indigo and azo dyes in semicontinuous reactors with long hydraulic retention time. Process Biochem 38 1213-1221... 

In a batch reactor, the first two terms in equation 12.2-1 are absent. In a semibatch reactor, one of these two terms is usually absent. In a semicontinuous reactor for a multiphase system, both flow terms may be absent for one phase and present for another. In a continuous reactor, the two terms are required to account for the continuous inflow to and outflow from the reactor, whether the system is single-phase or multiphase. 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

Figure 12.4 illustrates some modes of operation of semicontinuous reactors. In Figure 12.4(a), depicting a gas-liquid reaction of the type A(g) + B(f) - products, reactant A is dispersed (bubbled) continuously through a batch of reactant B an important example is an aerobic fermentation in which air (or 02) is supplied continuously to a liquid substrate (e.g., a batch of culture, as in penicillin production). In Figure 12.4(b),... 

Figure 12.4 Some modes of operation of semicontinuous reactors (a) gas-liquid reaction (b) gas-solid (catalyst or reactant) reaction (c) cyclic operation (reaction)-) and regeneration)- -)) for deactivating catalyst...

We focus mainly on the advantages and disadvantages of semibatch reactors. A semicontinuous reactor may be treated in many cases as either a batch reactor or a continuous reactor, depending on the overall kinetics and/or the phase of interest. 

A semicontinuous reactor for a fluid-solid reaction involves the axial flow of fluid downward through a fixed bed of solid particles, the same arrangement as for a fixed-bed catalytic reactor (see Figure 15.1(b)). The process is thus continuous with respect to the fluid and batch with respect to the solid (Section 12.4). 

Products from continuous, batch, and semicontinuous reactors can differ in a number of ways. Some of the more obvious characteristics that might be influenced are listed below ... 

Figure 6. Experimental data from semicontinuous reactor operated at 35°C and treating primary sludge from a municipal waste treatment plant [Data from 0 Rourke (5)]...

The reaction characteristic of the present system are best performed in a semicontinuous reactor in which the solid is stationary, as described in the previous section. This easily permits the two steps. In general, however, continuous reactors in which both the gas and solid phases move continuously are more important. We therefore briefly consider in this section the mathematical basis for the design of such a reactor. The chief reactor and operating parameters are gas and solids feed rates, product size distribution, bed size, and so on, and procedures for determining them are described. With a size distribution o(R), an elutriation stream and an arbitrary rate law for the changing particle size, a material balance on solids of size between R and R + dR yields... 

This procedure has been adapted to the design of the gas-solid reactor (i.e., step 2) of the present reaction. The solid-solid reaction of step 1 is performed in the semicontinuous reactor mentioned earlier, but enough q-phase is produced and stored for the continuous second step. The outlet particle size distribution from this step becomes the inlet distribution for the second step. The procedure already outlined above is employed to develop the final equation for exit product size distribution. 

A semicontinuous reactor (SCSTR) operation has most of the characteristics of a DCSTR in space, and the eft profile is typically as shown in Fig. 3.30 (see also Figs. 3.32 and 3.33). 

In a semicontinuous reactor in which monomers, surfactant, initiator, and water may be continuously fed into the reactor, emulsion polymerization does not follow the sequence of events described above. Thus, slow monomer feed and fast surfactant feed may lead to a system composed of polymer particles and micelles (Figure 6.3(a)). The system will contain only monomer-swoDen polymer particles if both monomer and surfactant are fed slowly (Figure 6.3(b)). On the other hand, a fast monomer feed and a low surfactant feed will lead to a system containing monomer droplets and polymer particles (Figure 6.3(c)). 

The first and second terms on the right-hand side of Eq. (69) should be removed for batch reactors, as well as for semicontinuous reactors to which no particles are fed. On the other hand, the coagulation terms may be neglected for stable formulations. Equations (69) and (70) are conveniently solved by using orthogonal collocation [119, 120]. 

Park et al. (1991) investigated a semicontinuous reactor with ethanol fed to maintain 20-30 g/1 for about 50 h when acetic acid accumulated to about 85-90 g/1 (see O Table 1.5). Oxidation of ethanol slowed to zero at this time, and the cell culture was rapidly recycled through a hollow fiber filter. The concentrated cells were then diluted with fresh medium and a new cycle started. It was found that the Acetobacter cells lost much viability, but the nonviable cells continued to oxidize ethanol during the later cycles. A high acetic acid output of about 85-90 g/1 was maintained through six cycles or for about 300 h. 

Although batch processes are the workhorse in research laboratory environments, continuous (and semicontinuous) reactors predominate for commercial PE production. In a continuous polymerization reactor, all monomers and reagents are constantly fed into the reactor, and the polymer is isolated from the effluent. Flows are adjusted to achieve the desired steady-state conditions as measured by online analytical instruments and polymer analysis. 

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